1. Field of the Invention
This invention relates to a method of recycling a spent flue gas denitration (de-NOx) catalyst and a method of determining a washing time of the spent flue gas denitration catalyst and, more particularly, to a method of recycling a spent flue gas denitration catalyst, of which activity has been lowered, using a washing liquid in a selective catalytic reduction (SCR) process for removing nitrogen oxides (NOx) contained in flue gas, and a method of determining a washing time using the washing liquid.
2. Related Art
A method of removing nitrogen oxides that is most widely used in thermal power stations, incinerators, industrial boilers, etc. is a selective catalytic reduction (SCR) technique, in which the nitrogen oxides react to ammonia used as a reducing agent under the presence of a catalyst to be decomposed into harmless nitrogen and water. The commercially available catalyst (SCR catalyst) is composed of a titanium dioxide (TiO2) carrier, vanadium (1 to 3%) and tungsten (10 to 20%) as active materials, and organic and inorganic binders for extrusion molding.
The activity of the flue gas denitration catalyst is degraded continuously with the passage of operation time due to poisoning by alkali metals, alkali earth metals and heavy metals contained in fly ash and due to deposition of solids, and thus its life span comes to an end after a predetermined period of time (about 2 to 5 years). Accordingly, extensive research aimed at recycling the spent flue gas denitration catalyst as well as at developing the flue gas denitration catalyst has continued to progress.
Korean Patent Application No. 2002-0071244 discloses a method of recycling a spent catalyst, in which the surface of the spent catalyst is washed with an alkali solution, polished with a polishing agent, washed repeatedly with an alkali solution such as NaOH, KOH and NH3 for 5 to 10 minutes, and finally washed with water. However, the above patent application does not disclose any method of calculating a washing time.
U.S. Pat. No. 4,615,991 discloses a method of washing and recovering a spent flue gas denitration catalyst, in which the spent flue gas denitration catalyst is washed with an aqueous oxalic acid solution and impregnated with a tungsten (W) compound. Here, the washing time is determined by the temperature of the washing liquid and a chemical quantitative analysis of the washing liquid and poisoning substances.
Especially, Raziyeh Kodayari et al. have proposed a method of recycling a spent flue gas denitration catalyst using H2SO4, VOSO4 and (NH4)WO4 solutions and a method of measuring poisoning substances contained in a washing liquid using a potassium electrode in Applied Catalysis B: Environmental 30 (2001), pp. 87-99.
As above, the conventional methods for calculating an optimal washing time analyze the poisoning substances contained in the washing liquid quantitatively using an analyzer such as an inductively coupled plasma atomic emission spectrometer; however, the convention methods using such analysis equipment have some drawbacks in that they requires a considerable amount of manpower and equipment and it takes a lot of time to carry out the analysis.